As one of sheet feeding-separating mechanisms that feeds paper toward an image forming section by separating sheet media (hereafter called sheet) stacked in a sheet feeding section one by one in an image formation apparatus such as a copier, a printer, a facsimile, or a printing machine, a mechanism of FRR (feed roller—reverse roller) system, so-called a backward separation system has been widely known as a high reliability one. This system performs a sufficient separating function for most of sheets.
In recent years, demands for color images in image formation apparatuses have been increasing. The paper used in these type of apparatuses is the so-called smooth paper that has a high degree of smoothness such as color copy paper used for color copying or paper for second original drawing has been used in many cases.
In these smooth papers, there is thin air layer between two sheets. Therefore, in addition to a frictional force between sheets, an adhesive force between sheets stronger than that of plain paper has been found to act on sheet separation capability when a sheet passes through the mechanism.
In case of the smooth paper having such a strong inter-sheet adhesive force, the sheets cannot properly be separated in the conventional FRR system, which may cause a double feed to occur. In addition, the inter-sheet adhesive force becomes also stronger in sheets with bad quality having a large amount of burrs caused by being improperly cut or in sheets such as second original drawing in which static electricity tend to develop, which may cause a double feed to occur.
To solve the problem, conventionally, a user manually loosens a set of sheets, which is one of the solutions. However, this increases user's work, or damage may be given to sheets depending on how to loosen them, which may cause a paper jam to occur due to folded ends of sheets.
As countermeasures of a machine side against these problems, there are some as disclosed in Japanese Patent Application Laid-Open No. 5-201571, Japanese Patent Application Laid-Open No. 5-213468, and Japanese Patent Application Laid-Open No. 5-330683, in which a separating part is vibrated to increase sheet separating capability. There are also some as disclosed in Japanese Patent Application Laid-Open No. 6-16271 and Japanese Patent Application Laid-Open No. 6-156764, in which sheets are vibrated before the separating part so that the sheets become easier to be separated to some extent before arriving the separating part.
However, any of the above-mentioned references discloses vibrating a component such as a separating part or a guide plate, therefore, there is a disadvantage that the apparatus becomes noisy. Moreover, when the separating part itself is vibrated, there may occur a skew in conveying quality. In addition, generating the vibrations before the sheets reach the separating part makes it difficult for the sheets to transport into the separating part, and a paper jam may occur in some cases.
Structures to adjust variably a pressure contact force between a reverse roller as a frictional separating roller and a feed roller are disclosed in Japanese Patent Application Laid-Open No. 6-263280 and Japanese Patent Application Laid-Open No. 8-59003. However, these inventions require adjusting operation to the pressure contact force whenever a type of paper or an environment is changed in order to obtain an initial separating capability, but the adjusting operation is complicated.
Further, as other countermeasures related to the mechanics, there is one that air is applied to spaces between sheets so that easier separation is prepared beforehand. In this case, a device to apply air is required, which makes the mechanism more complicated, and its arrangement is restricted.
More specifically, in the conventional FRR system, regarding the pressure contact force of the reverse roller against the feed roller, plain paper has no problem because there are a double-feed region where more than two sheets are fed without being separated and a misfeed region where no sheet is fed, and an appropriate region positioned between the double-feed region and the misfeed region is sufficiently wide. However, sheets having a strong inter-sheet adhesive force have a problem such that a double feed or a misfeed may occur due to the inter-sheet adhesive force because the appropriate region between the double-feed region and the misfeed region is narrowed.